The difficulties arising here will be described below with reference to the determination of the gas permeability of papers for the production of cigarettes on the one hand and the determination of the resistance to draw of cigarettes and filter rods on the other hand.
According to the draft of German Standard DIN 10251 of July, 1980, the static pressure difference between the two ends of the sample is designated as the resistance to draw of a cigarette or a filter rod if the sample is permeated by an airstream of 17.5 ml/s, measured at the outlet end, under constant conditions.
According to the draft of the International Standard ISO 2965-1979 published by the International Organisation for Standardisation, the air permeability of cigarette paper is defined as a ratio between the volumetric flow of air per unit area of the test material and the pressure difference between the two sides of the test material.
Various apparatuses for determining either the air permeability or the resistance to draw have been developed.
Thus, German Offenlegungsschrift No. 2 949200 discloses a device for determining the air permeability of cigarette paper in which several flow passages are arranged between the source of the volumetric flow and the test piece, which can be arranged in such a way as to alter the flow conditions. However, this results in a relatively complicated structure.
Furthermore, German Offenlegungsschrift No. 2 847 786 describes a process for determining the air permeability of cigarette or filter paper in which the volumetric flow through the test piece is adjusted to various values situated in the vicinity of a set value, thus for example at 17.5 ml/s. The respective pressure drop is then measured at the various adjusted values and the air permeability in turn calculated from the various measured values. Alternatively, various values in the vicinity of a predetermined pressure drop, thus for example 1 kPa, can be measured with the associated volumetric flows and the air permeability calculated therefrom. With this process, however, the air permeability can only be determined approximately for a specific volumetric flow or a specific pressure drop, by forming a mean value.
Finally, German Offenlegungsschrift No. 2 828 430 describes another device for determining the air permeability of cigarette paper in which the air is sucked through the cigarette paper via three different paths of flow which can each be sealed by valves. The path of flow permitting manual adjustment of the desired value must be determined in the first instance as a function of the air permeability of the cigarette paper. Only then can actual measurement take place.
Moreover, the known processes all have the limitation that they are specifically intended only for determining the air permeability of cigarette paper, the determination of the resistance to draw not being claimed.